Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) to increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WDM or other optical network is limited by absorption, scattering and other loss associated with the optical fiber. A signal-coding scheme employing bit-synchronous intensity modulation based on differential phase-shift keying has been proposed to suppress degradation induced by self-phase modulation (SPM), cross-phase modulation (XPM) and group velocity dispersion (GVD) in the optical fiber. In the scheme, the output from a laser diode is modulated with data using a phase modulator to generate an optical differential phase-shift key (DPSK) signal which is then remodulated with a clock using an intensity modulator. At the optical receiver, however, the remodulated DPSK signal must be both demodulated and converted to an intensity modulated signal for detection. Such a circuit for each channel may be bulky and expensive.